When did you leave home? Reconstructing juvenile life histories of coastal Washington adult coho salmon (Oncorhynchus kisutch) using otolith structure and chemistry

Coho salmon (Oncorhynchus kisutch) can express a range of juvenile life history strategies and understanding how they contribute to adult returns is critical for long-term harvest management and population resiliency in the light of environmental change. In this study we reconstructed the juvenile life histories of natural origin adult coho salmon returning to a lower and an upper basin tributary of the Chehalis River, in Washington state. First, we established the timing of otolith annulus formation for natural origin juveniles (n = 98) from the East Fork (EF) Satsop and the Newaukum River basins. We then used otolith microchemistry in conjunction with measurements of freshwater annulus location along the microchemistry transect from returning adults (n = 173) to describe the timing of freshwater emigration of fish from these two tributaries. Our results indicate that adult coho returning over two run years (2021 and 2022) exhibited seven unique successful juvenile life history strategies, three in the EF Satsop River and six in the Newaukum River. Emigration timing ranged from at least August through June and included individuals transitioning from age-0 to -1 and age-1 to -2. The majority (52-80%) of returning fish emigrated as age-1 smolts in the spring after their first otolith annulus had formed, however, 20-39% emigrated earlier, during active annulus formation (EF Satsop: October-April, Newaukum: October-December). A few individuals from the Newaukum River (2-3%) emigrated prior to the start of annulus formation, corresponding to a freshwater emigration before October. This is the first time successful juvenile migration strategies aside from spring age-1 smolts have been described for coho salmon in the Chehalis Basin. Identifying previously unrecognized life histories that make meaningful contributions to the spawning population highlights their impact on population stability and may help refine estimates of juvenile production, smolt-to-adult survival, and forecasting projections.